In a conventional rotary engine in which a rotor undergoes planetary rotation in a space which is defined by a trochoidal inner circumferential surface or a sliding surface of a rotor housing, and which has a major diameter portion and a minor diameter portion and is sealed by apex seals embedded in seal grooves disposed in the rotor along the rotation axis of the rotor, the apex seals undergo a centrifugal inertia which is increased along with the increase in the rate of rotor rotation, radially from the rotating center of the rotor. The apex seals also undergo a frictional force from the side wall surface of the groove due to the combustion gas pressure as the rotor rotates during operation of the rotary engine. The apex seal is urged against the sliding surface at the major diameter portion within the rotor housing by the outward centrifugal force from the rotating center of the rotor, whereas the apex seal is urged inwardly from the sliding surface of the rotor housing as the apex seal approaches the minor diameter portion in which an inertia is exerted in the direction opposite to that in the major diameter portion to separate the apex seal from the sliding surface of the rotor housing at the convexedly curved surface of the minor diameter portion. That is, the apex seal is floated in this case. In view of the above, for preventing the apex seal from separating (skipping) at the convexedly curved surface, an apex seal spring is disposed between the rotor and the apex seal for urging the apex seal toward the sliding surface of the rotor housing. However, since a frictional force is present between the apex seal and the side wall surface of the seal groove, there is a delay in the radial response of the apex seal due to the spring resiliency for the floating action exerted on the apex seal at the convexedly curved surface in the case when there is an abrupt change in the curved configuration of the sliding surface and, as a result, the apex seal is floated. Therefore, the gases compressed in the combustion chamber cause leakage, to worsen the fuel efficiency as compared with reciprocal engines.